Method for reducing process antenna effect

Abstract: A method of forming large contact pad areas for interconnection to an integrated circuit in a silicon substrate, wherein the large contact pad areas are at the periphery of the interconnection. A first layer of metal is formed on the substrate, and an insulating layer is formed over the first metal layer. At least one opening is formed to the first metal layer through the insulating layer, such that the exposed area of the first metal layer through the opening is effectively small to reduce the charge build-up at the gate oxide of the integrated circuit during subsequent plasma processing. A second layer of metal is deposited to form the large contact pad areas, such that the second metal layer electrically connects to the first metal layer through the opening in the insulating layer. Finally, a passivation layer is formed over the second metal layer. The first metal layer may be much smaller than the large contact pad area in order to further reduce the charge build-up at the gate oxide of the integrated circuit.

Description: The present invention relates to the manufacture of highly dense integrated circuits, and more particularly to the reduction of electrostatic charge build-up at the device during manufacture during, for example, plasma etching or the like process steps. In the manufacture of highly dense integrated circuits using Metal Oxide Semiconductor (MOS) technology with multiple metal layers, electrical charge may build up at the device gate oxide during plasma processing. The charge accumulates on floating polysilicon and metal layers electrically connected to the gate oxide. Interconnection metal lines act as an "antenna", amplifying the charging effect and leading to trapped charges at the gate oxide. These trapped charges can cause yield loss and reliability failures. Workers in the field have verified this problem experimentally. It has been found that the double metal layer technologies worsen the effect by the ratio of the antenna area to the gate oxide area. The worst degradation of gate oxide occurs during oxynitride deposition. However other plasma processing is also believed to lead to trapped charges at the gate oxide. Referring now to Figs. 1A and 1B, there are shown schematic diagrams of a prior process for connecting a large contact pad with an active MOS device by using a metal layer, which could lead to device damage due to the antenna effect. As can be seen in Fig. 1A, in the manufacture of MOS semiconductor devices, a large contact pad area 10 (of typical size 100 by 100 micrometers, and more than about 30 by 30 micrometers) is electrically connected by vias 12 to a polysilicon interconnection 14. The pad area 10 in a double metal process would consist of two layers 24 and 26 of metal, as shown in Fig. 1B. The upper metal layer, or metal II, 26 is connected through an insulating layer 28 to the lower metal layer, or metal I, 24 by a large via 25 which generally has a size less than 5 micrometers smaller than the large contact pad area 10. The polysilicon interconnection 14 is connected, by vias 16 in an insulating layer, to metal interconnection 18. This metal interconnection 18 is connected by vias 17 to polysilicon 20 and thence to active devices 22. There is a direct electrical connection between all elements, and charge accumulated during plasma processing on the polysilicon and large metal areas can lead to damage to the devices due to trapped charges. More specifically, charge will be induced and accumulated on the exposed lower metal layer 24 through the large via 25 in the pre-sputter etching step before the upper metal layer 26 is sputtered. The induced charge can lead to damage to the devices 22 via the direct electrical connection path comprising the lower metal layer 24, the polysilicon interconnection 14, the metal interconnection 18 and the polysilicon 20. In addition, the greater the size of the large via 25, the greater the amount of the charge induced and accumulated. It is common in the manufacture of integrated circuits today to use plasma etching, plasma enhanced chemical vapor deposition, and sputtering technologies. See VLSI Technology, S. M. Sze., Chapter 5 - Reactive Plasma Etching pages 184-189, Chapter 6 - Dielectric and Polysilicon Film Deposition pages 235-238, Chapter 9 - Metallization pages 386-391, published by McGraw-Hill International Editions 1988 in Singapore. A plasma is a collection of charged particles, including electrons and positive and negative ions, and it is these charged particles that collect along the conductive surfaces during plasma processing, and may produce the trapped charges damaging the gate oxide. It is a primary object of the present invention to provide a method that will reduce the charge that may accumulate at the gate oxide by reducing the area of the metal exposed during plasma processing. In Symp. VLSI Tech May 89, p73-74, Shone et al describe techniques for eliminating gate oxide charging. These techniques include subjecting the wafers to longer alloy cycles, depositing a thin dielectric layer on the wafer before oxynitride deposition, and attaching a substrate diode to the antenna. There is no suggestion to reduce gate oxide charging by reducing the exposed metal area. Moreover there is no disclosure of contact pads at the device periphery. GB-A-2174543 describes a package containing a semiconductor or integrated circuit chip having enlarged electrodes for soldering to bent electrically conductive leads so as to improve the reliability of the electrical connection. There is no teaching as to the elimination of the antenna effect. "Principles of CMOS VLSI Design" by N. Weste, Addison-Wesley 1985, pages 112-113, discloses a CMOS IC made by a double-level metal process and having contact pad areas at the periphery of an interconnection. EP-A-0 631 314 constitutes prior art for the present application under Article 54(3) EPC only. It discloses a similar double-metal structure to that achieved by the present invention, except that there is no disclosure of a silicon substrate, and in this prior art a passivation film covers the openings by which the upper and lower metal layers are connected. In accordance with the present invention, there is provided a method of reducing charge build up at the gate of an MOS device during processing in a plasma environment which is such as would normally produce charge build-up at the gate oxide which method comprises: forming a lower metal layer on a substrate; forming an insulating layer over the lower metal layer; and forming an upper metal layer over the lower metal layer such that the upper metal layer electrically connects to the lower metal layer through at least one opening in the insulating layer; characterised in that: the substrate is a silicon substrate; the upper metal layer constitutes a plurality of large contact pad areas for interconnection to an integrated circuit in the silicon substrate via the lower metal layer, each lower contact pad area being electrically connected to the lower metal layer via a plurality of said openings, and reduction of charge build-up at the gate oxide of the integrated circuit being achieved by reducing the area of the lower metal layer exposed through said openings; and in that a passivation layer is formed over the upper metal layer so as to leave exposed the part of the upper metal layer over said openings. In accordance with one embodiment of the present invention, an area of the lower metal layer to which each contact pad area is connected, is much smaller than the large contact pad area in order to further reduce the charge build-up at the gate oxide of the integrated circuit. In this case the area of the lower metal layer is about 6 x 100 micrometers, and the size of the large contact pad area is about 80 x 80 to 200 x 200 micrometers. In accordance with another embodiment of the present invention, the lower metal layer is a first metal layer and a second metal layer is formed so as to be the same size as the first metal layer so that the small second metal layer reduces the charge build-up; a second insulating layer is formed over the second metal layer; and a plurality of openings is formed to the second metal layer in the second insulating layer. A third layer of metal, i.e. the upper metal layer, is formed to constitute the large contact pad areas, such that the third metal layer electrically connects to the second metal layer, through the openings in the second insulating layer; and the passivation layer is formed over the third metal layer. Additional metal layers may be formed in a similar way. In accordance with a further embodiment of the present invention, the lower metal layer is a second metal layer and the upper metal layer is a third metal layer. In this case the method comprises the steps of forming a second insulating layer over the second metal layer; forming a plurality of openings to the second metal layer, through the second insulating layer; further processing in a plasma environment that would normally produce a charge build-up at the gate oxide of the integrated circuit, but wherein the charge build-up is reduced by having reduced the exposed area of the second metal layer; forming the third layer of metal to constitute the large contact pad areas, such that the third metal layer electrically connects to the second metal layer through the openings in the second insulating layer; and forming the passivation layer over the third metal layer. Additional metal layers may be formed in a similar way. The large contact pad areas are located at the periphery of the integrated circuit and the processing in the plasma environment includes plasma enhanced chemical vapor deposition, plasma etching or sputter etch. For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings, in which:- Figs. 1A and 1B illustrate a prior process for connecting a large contact pad with an active MOS device by using a metal layer, which could lead to device damage due to the antenna effect, Figs. 2A and 2B illustrate the method of the present invention for reducing the antenna effect in a double metal structure by using six small size vias between the first and second metal layers, Figs. 3A to 3C illustrate a further embodiment of the present invention in a double metal structure which reduces the antenna effect by using a much reduced contact pad area for the first metal layer, and Fig. 4 illustrates a further embodiment of the present invention, for a triple metal structure. Referring now to Fig. 2A, there is shown the preferred embodiment of the present invention. This embodiment is very similar to that of Fig. 1A. The only difference is that the present invention substitutes six small vias 30 for the single large via 25 of Fig. 1A. The small vias 30 expose a smaller area of lower metal I than the large via 25 does. The reduced area of exposed lower metal I lessens any charge accumulated during plasma processing. In this embodiment, the size of the vias 30 is about 2 x 2 micrometers. However, it should be understood that the number and size of the vias 30 are not intended to be limited to those described above. According to the present invention, the antenna effect can be effectively reduced so long as the exposed area of metal I is effectively decreased. Referring now to Fig. 2B, there is shown a cross-sectional representation of the embodiment shown in Fig. 2A. Field oxide (FOX) 34 is thermally grown on substrate 32, and a BPSG (Borophosilicate glass) insulating layer 36 is deposited on FOX 34. First metal layer 38 is deposited and patterned, and then another insulating layer 40 is formed. Vias 30 are opened by lithography and etching through the insulating layer 40. The size of the vias 30 is effectively decreased to minimize charge accumulation during plasma processing steps, so that the antenna effect is effectively reduced. Second metal layer 42 is then deposited and connected to the first metal layer 38. Passivation layer 44 is deposited over the second metal layer 42. Referring now to Fig. 3A, there is shown a further embodiment of the present invention. First metal layer 70 is deposited and patterned by conventional lithography and etching to cover a much smaller area than the eventual large contact pad area 72. The actual typical dimension of the much smaller area is about 6 x 100 micrometers, whereas the dimension of the larger contact pad area is between about 80 x 80 and 200 x 200 micrometers. The reduced area of metal 70 will further lessen any charge accumulated thereon during subsequent plasma processing. As in the prior art, the first metal layer 70 is connected to the polysilicon interconnection 14 through the vias 12. Referring now to Fig. 3B, second metal layer 80 is formed and connected to the first metal layer 70 through some small size vias 76, in order to reduce the sputter etch damage before metal 80 sputtering. The via size is, for example, about 2 x 2 micrometers. Passivation layer 82 is formed on top of second metal layer 80, around the perimeter of the contact pad. Referring now to Fig. 3C, there is shown a cross-sectional representation of the embodiment shown in Fig. 3B. Field oxide 90 is thermally grown on substrate 50, and BPSG insulating layer 92 is deposited on FOX 90. First metal layer 70 is deposited and patterned to be a much reduced area compared to the large contact pad area. This smaller area minimises charge accumulation during subsequent plasma processing steps. Second metal layer 80 is deposited and connected to the first metal layer 70, and comprises the complete pad area which is much larger than metal 70. Passivation layer 82 is deposited over the second metal layer 80. Referring now to Fig. 4, there is shown a cross-sectional representation of the formation of a contact pad in a three-metal technology. Field oxide 90 is grown on substrate 50, and BPSG insulating layer 92 is deposited on FOX 90. First metal layer 70 is deposited so as to have a much reduced area compared to the large contact pad area. This smaller area minimises charge accumulation during subsequent plasma processing steps. Second metal layer 80 is deposited and patterned to be the same size as the first metal layer 70, and is connected to the first metal layer 70. Further plasma processing may take place with minimal charge accumulation. Third metal layer 110 is deposited and connected to the second metal layer 80, and comprises the complete pad area. Passivation layer 112 is deposited over the third metal layer 110.

Claims: A method of reducing charge build up at the gate of an MOS device during processing in a plasma environment which is such as would normally produce charge build-up at the gate oxide which method comprises: forming a lower metal layer (38,70) on a substrate (32,50); forming an insulating layer (40) over the lower metal layer; and forming an upper metal layer (42,80) over the lower metal layer such that the upper metal layer electrically connects to the lower metal layer through at least one opening (30,76) in the insulating layer; characterised in that: the substrate is a silicon substrate; the upper metal layer constitutes a plurality of large contact pad areas (72,110) for interconnection to an integrated circuit in the silicon substrate via the lower metal layer (38,70), each contact pad area being electrically connected to the lower metal layer (38,70) via a plurality of said openings (30,76), and reduction of charge build-up at the gate oxide of the integrated circuit being achieved by reducing the area of the lower metal layer (38,70) exposed through said openings; and in that a passivation layer (44,82) is formed over the upper metal layer so as to leave exposed the part of the upper metal layer over said openings. The method as claimed in claim 1, wherein an area of the lower metal layer to which each contact pad area is connected, is much smaller than the large contact pad area in order to further reduce the charge build-up at the gate oxide of the integrated circuit. The method as claimed in claim 2, wherein said area of the lower metal layer is about 6 x 100 micrometers. The method as claimed in any of the preceding claims, wherein the size of each large contact pad area is from about 80 x 80 to about 200 x 200 micrometers. The method as claimed in any preceding claim, wherein the processing in the plasma environment is plasma enhanced chemical vapor deposition. The method as claimed in any of claims 1 to 4, wherein the processing in the plasma environment is plasma etching. The method as claimed in claim 6, wherein the plasma etching is a sputter etch. The method as claimed in claim 2, wherein said lower metal layer is a first metal layer (70), the method comprising the steps of forming a second metal layer (80) having the same size as the first metal layer; further processing in a plasma environment that would normally produce charge build-up at the gate oxide of the integrated circuit, but wherein the small second metal layer reduces the charge build up; forming a second insulating layer over the second metal layer; forming a plurality of openings to the second metal layer through the second insulating layer; forming a third layer (110) of metal to form the large contact pad areas, such that the third metal layer electrically connects to the second metal layer through the openings in the second insulating layer; and forming the passivation layer (112) over the third metal layer. The method as claimed in any one of claims 1 to 7, wherein the lower metal layer is a second metal layer, and the upper metal layer is a third metal layer.

Inventors: HSU BILL, KO JOE, HSU, BILL, KO, JOE
Applicants: UNITED MICROELECTRONICS CORP, UNITED MICROELECTRONICS CORPORATION
Application year: 1994
Main classification: H01L27
IPC codes: H01L23, H01L27